1. Field of the Invention
The invention relates to a method for regulating the vacuum supply of calibration tools, the tools including at least one dry calibration tool and at least one calibration bath, by which at least one vacuum pump is brought into contact with a calibration tool in order to draw air from the calibration tool, the pressure in the calibration tool being regulated to a target value by means of a control valve.
2. The Prior Art
An extrusion line for producing plastic profiles usually consists of an extruder with an extrusion die that is adjoined with calibration tools for calibrating and cooling the produced profile. Directly downstream of the extrusion die, what are termed dry calibration tools are mostly utilized, these generally being water-cooled metal blocs having a calibration port that conforms to the outer contour of the profile and that is laterally adjoined with vacuum slots. The vacuum slots communicate with vacuum pumps for pressing the profile against the wall of the calibration channel so that through intimate contact of the still soft profile with the wall of the calibration channel the profile be reliably formed into the desired shape on the one side and good heat transfer and, as a result thereof, fast cooling be achieved on the other side. The dry calibration tools are adjoined with calibration baths, which are vessels in which the now at least partially solidified profile is further cooled through direct contact with a coolant, calibration screens for guiding and keeping the profile dimensionally stable being disposed in said vessels. A negative pressure is also created in the vessels in order to ensure secure contact of the profile with the calibration screens. Such a solution is described in EP 0 659 536 A issued to the applicant of the present application.
Usually, each calibration tool is connected to one or a plurality of vacuum pumps in order to create the necessary negative pressure. The vacuum pumps must thereby be devised for maximum need which is obtained from the starting state, i.e., at production start, when the outer contour of the profile must for the first time be drawn to the tool's surface. The disadvantage thereof is that, in most of the operating conditions of the vacuum pumps, the vacuum produced is too high and has to be reduced in order to prevent damage to the profile. As vacuum pumps are generally very difficult to regulate, the pressure increase, i.e., the reduction of the negative pressure, on known tools is generally achieved by providing a control valve through which a vacuum chamber of the tool is in communication with the environment. When the pressure in the tool drops below a certain target value, the control valve opens to supply secondary air thus to keep the pressure within the desired range. Regulation may thereby be performed either automatically or manually. Such type solutions are disclosed in DE 33 01 556 C and in JP 05 131 525 A.
Although the method described permits to keep the pressure within a tool on a predetermined level, the energy expense is quite high. In modern calibration tools most of the energy required is used for supplying the vacuum.
It is the object of the present invention to reduce energy expense and to indicate a method of regulating the pressure by which significant energy savings are achieved without adding to the expense of the apparatus.
It is another object of the present invention to increase the accuracy with which the negative pressure is set in the discrete calibration tools in order to thus increase the quality of the profile produced.